The diverter valve regulates the excess pressure from the turbo, if the valve is broken and the excess pressure not regulated it can back up and stall the turbo.
As your engine relies on the turbo to give its air intake, if the turbo stalls you get inconsistent or no air for the air fuel mix. Hence your stuttery engine under load.
As for the blow off valve, there is no issue in fitting one of these but the turbo works much better if the excess air is fed back to the exhaust side of the turbo, sort of like an anti lag just not as good.
